1. Field of the Invention
This invention relates generally to a method and apparatus for recording and reproducing a digitized video signal on a magnetic tape and, more particularly, is directed to a method and apparatus for recording and reproducing a digitized video signal on a magnetic tape with a high recording density.
2. Description of the Prior Art
Conventionally, apparatus for recording and reproducing a video signal on a magnetic tape have been of the analog, rather than digital, type. However, there has been a recent turn torwards development of digital video tape recorders (VTR). Digital VTRs have a very high picture quality, which enables multiple generation dubbing or editing with virtually no picture impairment. Further, digital VTRs provide adjustment free circuits and self-diagnostic systems which enable easier maintenance and higher reliability.
With digital VTRs, an analog video signal is converted into digital form by an A/D (analog-to-digital) converter. In particular, the analog video signal is sampled by clock pulses having a sampling frequency which may be, for example, 4f.sub.sc, where f.sub.sc is the color sub-carrier frequency of the color video signal, resulting in the analog video signal being converted into a digitized video signal comprised of 8-bit words. The digitized signal is also coded by an error control encoder so that errors may be corrected and concealed on playback and, it is further coded by a channel encoder to achieve high density digital recording. The coded digitized signal is then recorded on a magnetic tape by means of a recording amplifier. However, it should be appreciated from the above that the recording bit rate, that is, the rate of occurrence of each bit of the digitized video signal, is extremely high. For example, in the above-described embodiment, where the color sub-carrier frequency f.sub.sc =3.58 MHz, the recording bit rate is equal to 4f.sub.sc times the number of bits per word. In other words, the recording bit rate is obtained as follows: EQU Bit rate=4.times.3.58.times.10.sup.6 .times.8=114.6 Mb/s.
Because of such high recording bit rate, the digitized video signal is not suitable for recording in a single recording channel.
Accordingly, it has been proposed to separate the digitized video signal into at least two separate channels prior to recording it on a magnetic tape so as to reduce the recording bit rate per channel. Typically, a magnetic head is associated with each channel and all of the magnetic heads are aligned to record the respective channels on a magnetic tape in parallel tracks extending obliquely on the tape, without guard bands between adjacent tracks and with the signals in alternate tracks being recorded with different azimuth angles. In this manner, the tape consumption using a digital VTR is not greater than that of an analog VTR. In order to separate the digitized video signal into, for example, two channels, an interface is provided which distributes successive 8-bit words of the digitized video signal into the respective channels by means of an identification signal added to the digital signal.
However, in such case where the digitized video signal is separated into a plurality of channels, each of the channels must separately process the video signal therein. This, of course, makes the circuitry complicated and expensive. For example, the interchanger that must be provided to distribute the reproduced signals to the correct channels during a special reproducing mode in which each head traces a plurality of record tracks, becomes complicated. Further, because of such complicated circuitry, it is inconvenient and troublesome when it is desired to perform any maintenance or check operation to ensure that the video signal is accurately reproduced.
Further, different VTRs vary in complexity. For example, a low-grade VTR, such as an ENG (electronic news gathering) machine, may use less channels than a standard VTR which, in turn, may use less channels than a high-grade mastering VTR. If common reproduction processing apparatus is provided for all of the above machines, such apparatus must include processing circuitry capable of being used with the most complex or high-grade mastering VTR. For example, if the high-grade mastering VTR uses sixteen channels, a complex interchanger must be provided for correctly separating the channels during a special reproducing mode. If it is desired to use a simple ENG machine, the same complex interchanger must be used. This, of course, is unnecessary.